17O NMR and 15N NMR chemical shifts of sterically-hindered amides: ground-state destabilization in amide electrophilicity.

The structure and spectroscopic properties of the amide bond are a topic of fundamental interest in chemistry and biology. Herein, we report 17O NMR and 15N NMR spectroscopic data for four series of sterically-hindered acyclic amides. Despite the utility of 17O NMR and 15N NMR spectroscopy, these methods are severely underutilized in the experimental determination of electronic properties of the amide bond. The data demonstrate that a combined use of 17O NMR and 15N NMR serves as a powerful tool in assessing electronic effects of the amide bond substitution as a measure of electrophilicity of the amide bond. Notably, we demonstrate that steric destabilization of the amide bond results in electronically-activated amides that are comparable in terms of electrophilicity to acyl fluorides and carboxylic acid anhydrides.